Use Multi-Project Pipelines with CE/EE and Omnibus

As per Slack conversation, using multi-project pipelines to better integrate building of gitlab-omnibus is not yet possible.

Developers on gitlab-ce are not allowed to run a pipeline on omnibus-gitlab since we now have #30634 so you need to have push/merge permissions on the target project.

They need to do that in order to have a Docker image with the latest changes that they can use to test locally. Now the process is done by a bot user that creates the image, but it should be done with MPP.

Not sure if this is the right project for this issue, in case let's move it to gitlab-ce or omnibus-gitlab.